Philly Brown does NOT have 4.5 speed. I don't know how in the hell he messed that up at the combine. That bomb he caught to set up the touchdown and the fake on the reverse shows his burst and acceleration.
These guys aren't track stars. Typically, bad form and starts hurt a 40 time. Thus, why I believe the event is severely overrated. In addition, some guys don't push it to the limit until game time (as oppose to workout warriors).